Helicoverpa zea is a significant Lepidopteran pest of major agricultural crops, including corn, cotton, and soy. Known as the corn earworm (CEW), cotton bollworm (CBW), and soy podworm (SPW), this polyphagous insect species is particularly difficult to control with insecticidal proteins from Bacillus thuringiensis (Bt) or other bacterial species. H. zea is considered at risk for resistance development to current insect control traits, given its ability to feed on many different crops and the current absence of a high-dose control strategy. Accordingly, new modes of action (MoA) are required to ensure the durability of transgenic plants protected from H. zea feeding damage.
The Cry1Da1 protein is a Lepidopteran-active protein that was first described by Hofte, et al. “Nucleotide sequence and deduced amino acid sequence of a new Lepidoptera-specific crystal protein gene from Bacillus thuringiensis.” Nucleic Acids Res. 18(18) (1990): 5545. This protein exhibits excellent insecticidal activity towards Spodoptera species including Spodoptera frugiperda (fall armyworm, FAW), a pest of several row crops, including corn, cotton and soybean. However, Cry1Da1 exhibits low-to-moderate activity towards a variety of other major Lepidopteran pests, including bollworms (e.g., Helicoverpa armigera and H. zea), borers (e.g., Ostrinia nubilalis and Diatraea grandiosella) and soybean looper (Pseudoplusia includens). Because of its narrow insecticidal spectrum and its inability to provide commercial-level protection against a range of important Lepidopteran agricultural pests such as CEW, the Cry1Da1 insecticidal protein has limited value as a transgenic plant insect control trait. As a result, no current commercial varieties of insect-protected crops utilize Cry1Da1 as a plant-incorporated protectant.
Despite its narrow insecticidal spectrum, Cry1Da1 is an interesting insecticidal protein because it appears that the Cry1Da1 protein uses an alternative MoA for controlling certain Lepidopteran pests. Evidence for this comes from studies with multiple resistant insect colonies. For example, field-derived colonies of Plutella xylostella (diamondback moth) and Pectinophora gossypiella (pink bollworm) that are resistant to Cry1Ac intoxication retain full sensitivity to the Cry1Da1 protein (Tabashnik, et al. “Cross-Resistance of Pink Bollworm (Pectinophora gossypiella) to Bacillus thuringiensis toxins.” Appl. Environ. Microbiol. 66 (2000): 4582-4584; Tabashnik, et al. “Cross-Resistance to Bacillus thuringiensis Toxin Cry1Ja in a Strain of Diamondback Moth Adapted to Artificial Diet.” J. Invert. Pathol. 76: (2000): 81-83.). These lines of evidence indicate that Cry1Da1 recognizes Lepidopteran midgut receptors distinct from those recognized by Lepidopteran-active proteins currently deployed in transgenic crops, including Cry1Ac, Cry1Ab, Cry1A.105, Cry1Fa, Cry2Ae, and Cry2Ab2. In view of this apparent novel MoA, optimization of Cry1Da1-like proteins for improved activity against a broader spectrum of Helicoverpa species while maintaining or increasing their insecticidal activity towards Spodoptera would create a high-value plant-incorporated protectant for insect resistance management.